Al-Baqarah 2:230 فان طلقها فلا تحل له من بعد حتى تنكح زوجا غيره فان طلقها فلا جناح عليهما ان يتراجعا ان ظنا ان يقيما حدود الله وتلك حدود الله يبينها لقوم يعلمون ٢٣٠

And if he hath divorced her (the third time), then she is not lawful unto him thereafter until she hath wedded another husband. Then if he (the other husband) divorce her it is no sin for both of them that they come together again if they consider that they are able to observe the limits of Allah. These are the limits of Allah. He manifesteth them for people who have knowledge.

And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him.¹ And if he [i.e., the latter husband] divorces her [or dies], there is no blame upon them [i.e., the woman and her former husband] for returning to each other if they think that they can keep [within] the limits of Allāh. These are the limits of Allāh, which He makes clear to a people who know [i.e.,understand].

So if a husband divorces his wife (irrevocably), He cannot, after that, re-marry her until after she has married another husband and He has divorced her. In that case there is no blame on either of them if they re-unite, provided they feel that they can keep the limits ordained by Allah. Such are the limits ordained by Allah, which He makes plain to those who understand.